1. Field of the Invention
The present invention relates to a recording method of a phase-change optical disk, namely, a rewritable optical disk. This method is suitably applied for a high volume optical medium, DVD+RW, or the like.
2. Description of the Related Art
Phase-change recording media, which are used for CD and DVD rewritable recording media have rapidly become very popular due to their high capacity, high-speed recording, and high compatibility with ROM (Read Only Memory). In recent years, it is required that recording/reproducing of mass image data is carried out at high speed, and higher speeds are being demanded of phase-change recording media. However, it is desirable that a high linear velocity recording disk which can be recorded at a high linear velocity, should be able to be recorded also in a low-speed drive for low linear velocity recording disks which record at a low linear velocity. This is possible with CD-R, which can record over a wide range of linear velocity.
However, in the case of the above-mentioned phase-change recording medium, it is difficult to perform recording over a wide range of linear velocity. In order to perform recording at a high linear velocity, a high power laser which outputs a high recording power is required. The recording power of the laser light used in a low-speed drive is usually lower than the recording power of the laser light output in a high-speed drive for high linear velocity recording. Hence, it is difficult to record a high linear velocity recording disk in a low-speed drive.
The aforesaid phase-change recording medium is usually optimized for recording at a high linear velocity. In the case of a phase-change recording medium designed in this way, the recording power to record is higher than the optimal recording power for a low recording linear velocity. Thus, in order to perform recording at a lower recording power with this phase-change optical recording medium, the sensitivity of this phase-change optical recording medium must be increased. In order to increase sensitivity of this phase-change optical recording medium, the reflectance of this phase-change optical recording medium can be lowered. However, when designing this phase-change optical recording medium as a DVD, it is necessary to maintain compatibility with DVD-ROM. Thus, the above-mentioned reflectance cannot be made lower as desired.
The highest recording linear velocity in rewritable DVD currently commercialized in the past several years is 2.4×. A phase-change optical recording medium, which can be recorded at a higher recording linear velocity than 2.4× and also in a low-speed drive, namely, which is downward compatible, has not yet been provided.
In order to provide downward compatibility, it is required to select the composition of the above-mentioned phase-change optical recording medium and the material of the recording layer, and optimize the recording conditions of this phase-change optical recording medium, so that it is recordable at a low recording power and the recording power margin is large.
In the prior art, for example in Japanese Patent (JP-B) No. 3124720 or Japanese Patent Application Laid-Open (JP-A) No. 2000-322740, by controlling the pulse-width of a laser pulse-like waveform, it can be made CAV (Constant Angular Velocity) recording possible at 2.4×. In the case of rewritable DVD, however, there is a problem that it is difficult to realize a recording linear velocity higher than 2.4× together with downward compatibility so that recording can also be performed in a low-speed drive.
In JP-B No. 2844996, for example, instead of using a fixed erasing power for high speed recording, a method of modulating the erasing power by the reproduction power range is disclosed. However, in the case of this method, a sufficient erasure cannot be performed, and there is the problem that an amorphous phase may be formed depending on the level of the erasing power.
Also, for example, in JP-B No. No. 2941703, a method wherein a rear edge cooling pulse interval is basically eliminated when forming a record mark, is described. However, in the case of this method, there is a problem that it is difficult to form a record mark of predetermined length.
In the case of DVD, such phase-change optical recording medium and recording method thereof are required that recording can be performed at a recording linear velocity as fast as 4× (as 1× linear velocity is 3.49 m/s, this is approx. 14 m/s (13.96 m/s)), and also with a recording power below the optimal recording power of a phase-change recording medium for recording of a recording linear velocity of 1× to 2.4×.
In this regard, it is important to optimize the crystallization rate of the recording layer in the phase-change optical recording medium. In the recording layer, the overwrite characteristics, particular the characteristics of the first overwrite, deteriorate when recording is performed at a high linear velocity. Hence, to enable recording at a high linear velocity, it is important to optimize the elements and elemental composition of this recording layer so that the crystallization rate in this recording layer increases.
In order to form a record mark (amorphous phase) in the aforesaid recording layer in the case of the aforesaid phase-change optical recording medium, it is necessary to heat the material of this recording layer to near the melting point thereof and to perform quenching in a short time. The crystallization rate in the recording layer is larger, the larger is the temperature gradient over time, and the longer is the non-heating time (cooling time) required to suppress recrystallization is longer. However there is a limit to the heating time and cooling time. During recording at a high linear velocity where it is difficult to raise the temperature in a short time, therefore, the recording power must be increased. Also when recording at a low linear velocity, the aforesaid crystallization rate in the aforesaid recording layer is large, so the recording power must likewise be increased.
Accordingly, in a phase-change optical recording medium for recording at a high linear velocity, the aforesaid crystallization rate cannot be made too high. Consequently, if a fixed erasing power is irradiated, an amorphous phase is easily formed even if the erasing power is not so high, and the erasing power cannot be increased too much, the higher is the linear velocity. For this reason, the above-mentioned crystallization rate may be optimized at an intermediate linear velocity between a low linear velocity and a high linear velocity. However, in this case, the erasing power is not sufficient for a high linear velocity, and mark erasure properties during overwrite are poor.